Ferromagnetic liquids commonly are referred to as ferrofluids and typically comprise a colloidal dispersion of finely-divided magnetic particles, such as iron, .alpha.Fe.sub.2 O.sub.3 (hematite), magnetite and combinations thereof, of subdomain size, such as, for example, 10 to 800 Angstroms, and more particularly 50 to 500 Angstroms, dispersed in a liquid through the use of a surfactant-type material. Typically ferrofluids are remarkably unaffected by the presence of applied magnetic fields or by other force fields, and the magnetic particles remain uniformly dispersed throughout the liquid carrier. Ferrofluid compositions are widely known, and typical ferrofluid compositions are described, for example, in U.S. Pat. Nos. 3,700,595, issued Oct. 24, 1972, and 3,764,540, issued Oct. 9, 1973, while a particular process for preparing such ferrofluid compositions is described more particularly in U.S. Pat. No. 3,917,538, issued Nov. 4, 1975, which describes a grinding or ball-mill technique for preparing ferrofluid compositions, and U.S. Pat. No. 4,019,994, issued Apr. 26, 1977, describing more particularly a precipitation technique for preparing ferrofluid compositions.
Ferrofluids have been suggested to be prepared using a wide variety of liquid carriers. However, current state-of-the-art ferrofluids typically employ a hydrocarbon carrier or, for example, a diester liquid, such as ethyl-hexyl azilate. Liquid ferrofluids typically comprise a dispersion of colloidal magnetite stabilized by an aliphatic surfactant in a hydrocarbon-liquid carrier, such as, for example, the use of an oleic-acid-type surfactant. These diester ferrofluids have found use in audio-voice-coil-damping and inertia-damping apparatus and for use in bearings and seals. However, the hydrocarbon-based, and particularly the diester-based ferrofluids have been limited in some applications, paricularly in seals, because of a relatively high vapor pressure of the carrier of greater than about 10.sup.-4 torr at 20.degree. C.
Ferrofluids have been employed in single and multistage seals (see, for example, U.S. Pat. No. 3,620,584, issued Nov. 16, 1971), which seals are used in the vacuum industry. Attempts to provide stable, low-vapor-pressure, ferrofluid compositions, particularly for use at 10.sup.-4 torr or more, particularly equal to or less than 10.sup.-7 torr, have not been commercially successful. Accordingly, there exists a need for stable, low-vapor-pressure ferrofluids for seals and other uses and for a method of manufacturing such ferrofluid compositions.